Process for preparing organically modified aerogels using alcohols, wherein the resultant salts are precipitated

ABSTRACT

The invention concerns a process for preparing organically modified aerogels, in which a) a silicic acid sol with a pH ≦4.0 is produced from an aqueous potassium silicate solution using at least one organic and/or inorganic acid; b) the resultant silicic acid sol is polycondensed by the addition of a base to form SiO 2  gel; c) the gel produced in step b) is washed with an organic solvent until the water content of the gel is ≦5 wt %; d) the gel obtained in step c) is surface-modified with at least one C 1-6  alcohol; and e) the surface-modified which are difficult to dissolve in the silicic acid sol. Before step b), the resultant salts, which are difficult to dissolve, are precipitated to the greates possible extent and separated from the silicic acid sol.

Process for the preparation of organically modified aerogels by usingalcohols, in which process the salts formed are precipitated.

The invention relates to a process for the preparation of organicallymodified SiO₂ aerogels with the use of alcohols, in which process thesalts formed are precipitated.

BACKGROUND OF THE INVENTION

1. Field of the Invention

Aerogels, particularly those having porosities over 60% and densitiesbelow 0.6 g/cm³ have an extremely low thermal conductivity and for thisreason are used as thermal insulating materials, as described e.g. inEP-A-0 171 722.

2. Discription of the Related Art

Aerogels in the broader sense of the term, i.e. in the sense of a "gelwith air as dispersing agent," are prepared by drying a suitable gel.Understood by the term "aerogel" in this sense are aerogels consideredin the narrow sense, xerogels and cryogels. A dried gel is considered anaerogel in the narrow sense of the term when the liquid of the gel isremoved at temperatures above the critical temperature and starting frompressures above the critical pressure. However, if the liquid of the gelis removed under subcritical conditions, e.g. with the formation of aliquid-vapor boundary phase, then the resulting gel is designated as axerogel. It should be noted that the gels according to the invention areaerogels in the sense of gels with air as dispersing agent.

SO₂ aerogels can be prepared e.g. by acid hydrolysis of tetraethylorthosilicate in ethanol. During the hydrolysis a gel is formed whosestructure is determined, among other things, by the temperature, the pHand the duration of the gelation process. However, during the drying ofthe wet gel the gel structure generally collapses because the capillaryforces resulting during drying are extremely great. Collapse of the gelcan be prevented by carrying out the drying above the criticaltemperature and critical pressure of the solvent. Since in this rangethe liquid/gas phase boundary disappears, the capillary forces alsovanish and the gel does not change during the drying process, i.e. noshrinking of the gel during drying will occur, either. Methods ofpreparation based on this drying technology are disclosed e.g. in EP-A-0396 076 or WO 92/03378. However, e.g. when ethanol is used, thistechnique requires a temperature of about 240° C. and pressures over 60bar. Although the exchange of ethanol against CO₂ before drying doesreduce the drying temperature to about 30° C., the pressure required isthen over 70 bar.

An alternative for the above drying method is offered by a process ofsubcritical drying of SiO₂ gels, if, before drying, the latter arereacted with a chlorine-containing silylating agent. In that case theSiO₂ gel can be obtained e.g. by acid hydrolysis of tetraalkoxysilanes,preferably tetraethoxysilane (TEOS) in a suitable solvent, preferablyethanol, by means of water. In a further step, after exchange of thesolvent against a suitable organic solvent, the resulting gel is reactedwith a chlorine-containing silylating agent. Used as silylating agents,because of their reactivity, are preferably methylchlorosilanes(Me_(4-n) SiCl_(n), with n=1 to 3). Thereupon the resulting SiO₂ gelwhose surface has been modified by methylsilyl groups, can be dried inair from an organic solvent. In this way aerogels having densities ofless than 0.4 g/cm³ and porosities over 60% can be obtained. WO 94/25149gives a detailed description of the method of preparation based on thisdrying technique.

Furthermore, before drying, the above-described gels can be treated inthe aqueous alcoholic solution with tetraalkoxysilanes, and then aged,in order to increase the strength of the gel network, as disclosed e.g.in WO 92/20623.

However, the tetraalkoxysilanes used as starting materials in theabove-described process are extremely expensive. Furthermore, duringsilylation with chlorine-containing silylating agents hydrogen chloride(HCL) and a plurality of side products associated therewith willnecessarily form, which in some cases require a very expensive andcost-intensive purification of the silylated SiO₂ gels by repeatedwashing with a suitable organic solvent. The particularlycorrosion-resistant installations required in this operation are alsovery expensive. The safety risks associated with the formation of verylarge amounts of HCl gas will additionally require a very involvedtechnique, and is thus also very cost-intensive.

A first, not inconsiderable cost reduction can be achieved by usingwater glass as the starting material for the preparation of the SiO₂gels. To this end, a silicic acid can be prepared from an aqueous waterglass solution with the aid of ion exchanger resins, which acid willpolycondense to a SiO₂ gel upon the addition of a base. Then in afurther step, after exchange of the aqueous medium against a suitableorganic solvent, the resulting gel is reacted with a chlorine-containingsilylating agent. Used as silylating agents, because of theirreactivity, are preferably methylchlorosilanes (Me_(4-n) SiCl_(n) withn=1 to 3). The resulting SiO₂ gel surface-modified with methylsilylgroups can then also be dried in air from an organic solvent. The methodof preparation based on this technique is described e.g. in DE-A-43 42548.

However the above-described problems of extremely high production costsassociated with the use of chlorine-containing silylating agents are notsolved by the use of water glass as starting material.

German Patent Application P 19502453.2 describes the use of achlorine-free silylating agent. This method starts out from thesilicate-type lyogel obtained with the above-described process bydifferent methods, and reacted with a chlorine-free silylating agent.Preferably used in this case as silylating agents aremethylisopropenoxysilanes (Me_(4-n) Si(OC(CH₃)CH₂)_(n) with n=1 to 3).Thereupon, the thus resulting SiO₂ gel surface-modified with methylsilylgroups can again be dried in air from an organic solvent.

Although the use of chlorine-free silylating agents will solve theproblem of HCl formation, the chlorine-free silylating agents used alsorepresent an extremely high cost factor.

WO 95/06617 discloses hydrophobic silicic acid aerogels obtainable bythe reaction of a water glass solution with an acid at a pH of from 7.5to 11, extensive freeing of the resulting silicic acid hydrogel fromionic components by washing with water or dilute aqueous solutions ofinorganic bases, --with the pH of the hydrogel maintained in the rangeof 7.5 to 11--displacement of the aqueous phase contained in thehydrogel by an alcohol, and subsequent supercritical drying of theresulting alcogel.

In this process suitable alcohols for the water exchange are C₁ -C₅alcohols, preferably C₃ -C₅ alcohols, and isopropanol in particular.

It is known that when the above-mentioned alcohols are used undersupercritical conditions (WO 95/06617), esterification of the alcoholswith the surface OH groups of the lyogel will take place. As a result,alkoxy-modified aerogels, e.g. isopropoxy-modified aerogels areobtained, which have hydrophobic surface groups.

However, a disadvantageous aspect of the method of preparation disclosedin WO 95/06617 is that the drying requires supercritical conditionswhich, e.g. for isopropanol, are at a temperature in the range of 240 to280° C. and at a pressure of about 55 to 90 bar.

OBJECTS OF THE INVENTION

A further unsolved problem are the aqueous salt solutions which areobtained in the preparation of aerogels from water glass. In order toconvert a water glass solution into a silicic acid sol capable ofcondensation, the cations (mostly sodium and/or potassium ions) must beexchanged against protons in the water glass solution. For this purpose,organic and/or inorganic acids may be used. The salts of theabove-mentioned cations (e.g. NaCl or Na₂ SO₄) which will alsonecessarily be formed in the dissolved state must be washed out from thegel before, during or after gel aging. At present, these highly diluteaqueous salt solutions constitute a great disposal problem, because theycan no longer be discharged into rivers or lakes in relatively largequantities. A final disposal which meets current regulations representsan extremely high cost factor.

Hence the object of the present invention was to provide a process forthe preparation of aerogels having hydrophobic surface groups, a processwhich does not have any of the above-described problems known in theprior art. In particular, the process according to the invention shouldbe economical and capable of being carried out in a technically simplemanner.

A further object of the present invention was to provide a process forthe preparation of organically modified SiO₂ aerogels in which no diluteaqueous salt solutions will be formed.

SUMMARY OF THE INVENTION

These objects are met by a process for the preparation of organicallymodified aerogels, comprising

a) the preparation of a silicic acid sol having a pH of ≦4.0 from anaqueous water glass solution with the aid of at least one organic and/orinorganic acid;

b) polycondensation of the resulting silicic acid sol to a SiO₂ gel bythe addition of a base;

c) washing the gel obtained in Step b) with an organic solvent until thewater content of the gel is ≦5% by weight;

d) modifying the surface of the gel obtained in Step c) with at leastone C₁ -C₆ alcohol; and

e) drying the surface-modified gel obtained in Step d), characterized inthat at least one acid forms difficultly soluble salts with the cationsof the water glass in the silicic acid sol, and that before Step b), theresulting difficultly soluble salts are extensively precipitated andseparated from the silicic acid sol.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Used as water glass solution in Step a) is generally a 6 to 25% byweight (calculated on the SiO₂ content) sodium and/or potassium waterglass solution. A 17 to 20% by weight water glass solution is preferred.Furthermore, the water glass solution may also contain up to 90% byweight (calculated on SiO₂) of zirconium, aluminum and/or titaniumcompounds capable of condensation.

The acids used are generally 15 to 50% by weight acids, which formdifficultly soluble salts with sodium and/or potassium ions. Mixtures ofsuitable acids can also be used. Sulfuric, phosphoric, hydrofluoric andoxalic acid are preferred. Sulfuric acid is especially preferred.

To achieve an as complete precipitation and good separation of thedifficultly soluble salts formed in Step a) as possible, the silicicacid sol should have a temperature between 0 and 30° C., preferablybetween 0 and 15° C., and by particular preference between 0 and 5° C.This can be achieved by bringing the water glass solution, the acidand/or the silicic acid sol to a temperature between 0 and 30° C.,preferably between 0 and 15° C., and by particular preference between 0and 5° C. If, in so doing, a supersaturated salt solution should form,the salt can be precipitated by suitable seeding with appropriateseeding crystals. The salts formed are separated by means known topersons skilled in the art, e.g. by filtration, suction filtration,membranes or crystallization vessels. Semi-continuous or continuousprocesses are preferred.

After the salts have been separated off, the silicic acid sol isadjusted with water to a concentration between 5 and 12% by weight(calculated on the SiO₂ content). A 6 to 9% by weight silicic acidsolution is particularly preferred.

The polycondensation of the silicic acid sol obtained in Step a) to forma SiO₂ gel takes place in Step b) by the addition of a base in a pHregion of between 3.0 and 7.0, preferably between 4.0 and 6.0. Used asbase is generally NH₄ OH, NaOH, KOH, Al(OH) ₃, colloidal silica and/oran alkaline water glass solution. NH₄ OH, NaOH and KOH are preferred,with NaOH especially preferred. Mixtures of the aforementioned can alsobe used.

Step b) is generally carried out at a temperature between the freezingpoint of the solution and 100° C. Optionally, a shaping step, such asspray forming, extrusion or drop formation can simultaneously be carriedout.

Before Step c) the gel is preferably aged, an operation generallycarried out at 40 to 100° C., preferably at 80 to 100° C., and at a pHof 4 to 11, preferably 5 to 7. The duration of this operation isgenerally 1 second to 12 hours, preferably 1 second to 5 hours.

Optionally, the aged hydrogel can be washed with water until it is freeof electrolytes.

In Step c), the gel obtained in Step b) is washed with an organicsolvent until the water content of the gel is ≦5% by weight, preferably≦2% by weight, and by particular preference ≦1%. Used as solvent aregenerally aliphatic alcohols, ethers, esters or ketones, as well asaliphatic or aromatic hydrocarbons.

Preferred solvents are C₁ -C₅ alcohols, acetone, tetrahydrofuran, ethylacetate, dioxane, n-hexane and toluene. Particularly preferred solventsare isopropanol, isobutanol, tert.-butanol and acetone. Mixtures of theaforementioned solvents can also be used. Furthermore, the water canfirst be washed out with a water-miscible alcohol, and the latter canthen be washed out with a hydrocarbon.

Furthermore, the gel obtained in Step c) can additionally be subjectedto a solvent exchange. In case of solvent exchange the same solvents maybe used, in principle, as in the washing operation of Step c). Thesolvents preferred for washing are preferred also for the solventexchange.

Understood by the term "solvent exchange" is not only a one-timeexchange of the solvent, but also an optional multiple repetition withdifferent solvents.

The lyogel obtained in Step c) can also be subjected to a further agingprocess. This is generally done between 20° C. and the boiling point ofthe organic solvent. Optionally, the aging may also be carried out alsounder pressure at elevated temperatures. The duration is generally 1second to 48 hours, preferably 1 second to 24 hours. Such an aging canoptionally be followed by a further solvent exchange with the same ordifferent solvent. This additional aging step may optionally be repeatedseveral times.

In Step d) the gel obtained in Step c) is surface-modified with at leastone C₁ -C₆ alcohol ion such a way that it is kept in a pressure vesselor autoclave under pressure and elevated temperature.

C₃ -C₅ alcohols, such as isopropanol, isobutanol, tert.-butanol,sec.-pentanol and tert.-pentanol are preferentially used. Isopropanol,isobutanol and tert.-butanol are particularly preferred.

In so doing, the alcohol is generally used in an amount of from 1 to100% by weight, calculated on the total amount of solvent.

The alcohols may be used alone, in mixtures or with other nonreactiveorganic solvents or solvent mixtures, such as acetone, tetrahydtrofuran,dioxane, n-hexane or toluene.

The temperatures and pressures for surface modification depend on therespective solvent or solvent mixture used. However they are clearlybelow the critical temperature and critical pressure of the alcoholsused.

A temperature between 25° C. and 220° C., and by particular preferencebetween 150° C. and 220° C. is used.

The pressure is preferably between 1 and 50 bar, and by particularpreference between 20 and 50 bar,

The times during which the lyogel is maintained under these conditionsare preferably between 30 minutes and 20 hours, and by particularpreference between 30 minutes and 10 hours.

Small amounts of a silylating agent may be optionally be added. Suitableas silylating agents are generally silanes of formulas R₁ _(4-n)SiCl_(n) or R_(14-n) Si (OR²)_(n) (with n =1 to 3), where R₁ and R²,independently of one another, are C₁ -C₆ -alkyl, cycloalkyl or phenyl.Isopropenoxysilanes and silazanes are also suitable.Trimethylchlorosilane is preferably used. Furthermore, all silylatingagents known to persons skilled in the art may be employed, e.g. eventhose disclosed in DE-A-44 30 669.

The quantities are generally between 0 and 1% by weight (calculated onthe lyogel); the concentrations are preferably between 0 and 0.5% byweight, and with particular preference between 0 and 0.2% by weight.

To speed up the surface-modifying process, water may additionally bepresent in the system In that case concentrations between 0 and 10% byweight (calculated on the lyogel) are preferred. Moreover, to speed upthe process, catalysts known to persons skilled in the art, such asacids, bases or organometallic compounds, may also be present in thesystem

Optionally, the surface-modified gel obtained in Step d) may besubjected to a solvent exchange before Step e). Solvents generally usedfor this purpose are aliphatic alcohols, ethers, esters or ketones, aswell as aliphatic or aromatic hydrocarbons. Mixtures of theaforementioned solvents may also be used. Preferred solvents aremethanol, ethanol, i-propanol, acetone, tetrahydrofuran, ethyl acetate,dioxane, n-hexane, n-heptane and toluene. Particularly preferred, assolvent, is i-propanol.

In Step e), the surface-modified and preferably after-washed gel isdried under subcritical conditions, preferably at temperatures of from-30° C. to 200° C., and particularly between 0 to 100° C. The pressuresused for drying are preferably between 0.001 to 20 bar, and byparticular preference between 0.01 and 5 bar.

The gel obtained in Step d) may be dried also under supercriticalconditions. Depending on the solvent used, this requires temperatureshigher than 200° C. and/or pressures higher than 20 bar. This ispossible without any problems, but is more expensive and affords nosignificant advantages.

In general, the drying is continued until the gel has a residual solventcontent of less than 0.1% by weight.

In another embodiment the gel may, after the shaping polycondensation inStep b) and/or in any subsequent step, be comminuted by techniques knownto persons skilled in the art, e.g. by grinding.

Furthermore, in order to reduce the contribution of radiation to thermalconductivity, the gel may be treated before the gel preparation withIR-opacifying agents, such as carbon black, titanium oxide, iron oxidesand/or zirconium oxides.

Furthermore, it is possible to treat the sol with fibers beforepreparation of the gel, in order to increase its mechanical stability.Suitable for use as fiber materials are inorganic fibers such as glassfibers or mineral fibers, organic fibers such as polyester fibers,aramide fibers, Nylon fibers or fibers of vegetable origin, as well asmixtures thereof. The fibers may also be coated, e.g. polyester fibersmetallized with a metal such as aluminum.

In another embodiment the gel, depending on its use, may be subjectedbefore surface modification to an additional network reinforcement. Thisis done by reacting the resulting gel with a solution of an alkyl and/oraryl orthosilicate capable of condensation and having the formula R¹_(4-n) Si (OR²)_(n), where n=2 to 4, and R₁ and R², independently of oneanother, are linear or branched C₁ -C₆ -alkyl groups, cyclohexyl groupsor phenyl groups, or with an aqueous silicic acid solution. This networkreinforcement can be carried out before and/or after every aging step orsolvent exchange.

In another preferred embodiment the gel has, before drying, an E-modulusof more than 3 MPa, a BET surface area of more than 400 m² /g and a poreradius distribution in the range of from 2 to 20 nm, preferably in therange of from 5 to 10 nm, so that the aerogels obtained aftersubcritical drying preferably have a density of ≦200 kg/m³, and byparticular preference a density of ≦150 kg/m³.

Below, the process according to the invention is described in greaterdetail by means of an embodiment, without thereby limiting said processin any way.

EXAMPLE 1

236 g of 25% H₂ SO₄ cooled to 0° C. is dropwise treated, undercontinuous cooling to 0° C., with 707 g of a sodium water glass solutioncooled to 7° C. (containing 17% by weight of SiO₂ and a Na₂ O:SiO₂ ratioof 1:3.3). A pH of 1.6 is obtained. The precipitating Na₂ SO₄.10 H₂ O isseparated at 0° C. from the silicic acid sol by suction filtration, andthe silicic acid sol is diluted with 280 mL of H₂ O. The resultingsilicic acid sol is treated at 5° C. and under stirring with 26 mL of a1 N NaOH solution, to bring the pH to 4.7. The resulting hydrogel isthen aged for 2.5 hours at 85° C.

The modulus of elasticity of the aged hydrogel is 15.5 MPa. It is washedwith 2 L of warm water and then extracted with isopropanol, until thewater content of the gel is below 2.0% by weight. Theisopropanol-containing lyogel is then heated in isopropanol in anautoclave to 220° C. and a pressure of 40 bar, and maintained underthese conditions for 3 hours. The gel is dried in air (3 hours at 40°C., then 2 hours at 50° C. and 12 hours at 150° C.).

The resulting transparent aerogel has a density of 0.15 g/cm³. Itsspecific surface area according to BET is 500 m² /g. The λ value is0.018 W/mK.

The thermal conductivity was measured by a hot wire method (see e.g. 0.Nielsson, G. Ruschenpohler, J. Gross and J. Fricke, HighTemperatures--High Pressures, Vol. 21, 267-274 (1989)).

What is claimed is:
 1. Process for the preparation of organicallymodified aerogels, comprisinga) the preparation of a silicic acid sol ofpH ≦4.0 from an aqueous water glass solution by means of at least oneorganic and/or inorganic acid, with at least one acid formingdifficultly soluble salts at temperatures between 0° C. and 30° C. withcations of the water glass in the silicic acid sol; b) extensivelyprecipitating the resulting difficultly soluble salts at temperaturesbetween 0° C. and 30° C., and separating them from the silicic acid sol;c) polycondensing the resulting silicic acid sol by the addition of abase, to form a SiO₂ gel; d) washing the gel obtained in Step c) with anorganic solvent until the water content of the gel is ≦5% by weight; e)modifying the surface of the gel obtained in Step d) with at least oneC₁ -C₆ alcohol; and f) drying the surface-modified gel obtained in Stepe).
 2. Process according to claim 1 wherein the water glass solution inStep a) is a 6 to 25% by weight sodium and/or potassium water glasssolution.
 3. Process according to claim 1 wherein said water glasssolution contains up to 90% (calculated on SiO₂) zirconium, aluminumand/or titanium compounds capable of condensation.
 4. Process accordingto claim 1 wherein said acid of step (a) is 15 to 50% by weight sulfuricacid.
 5. Process according to claim 1 wherein the resulting silicic acidsol has a temperature in the range of from 0 to 30° C.
 6. Processaccording to claim 1 wherein said base in Step c) is NaOH, NH₄ OH,Al(OH)₃, colloidal silica, and/or an alkaline water glass solution. 7.Process according to claim 1 wherein the gel is aged before Step d) at40 to 100° C. and at a pH of 4 to 11 for a period between 1 second and12 hours.
 8. Process according to claim 1 wherein said organic solventin Step d) is a C_(1-C) ₅ -alcohol, acetone, tetrahydrofuran, ethylacetate, dioxane, n-hexane, n-heptane and/or toluene.
 9. Processaccording to claim 1 wherein before surface modification, the gelobtained in Step c) is reacted with a solution of an alkyl and/or arylorthosilicate capable of condensation and having the formula, R₁ _(4-n)Si(OR²)_(n), where n=2 to 4, and R₁ and R², independently of one anotherare linear or branched C₁ -C₆ -alkyl groups, cyclohexyl groups or phenylgroups, or with an aqueous silicic acid solution.
 10. Process accordingto claim 1 wherein said alcohol in Step e) is at least one C₃ -C₅alcohol.
 11. Process according to claim 1 wherein said alcohol in Stepe) is isobutanol or tert.-butanol.
 12. Process according to claim 1,wherein the alcohol is used in an amount of from 1 to 100% by weight,calculated on the whole amount of solvent.
 13. Process according toclaim 1, wherein the surface modification is carried out at atemperature in the range of from 25° C. to 220° C. and a pressure offrom 1 to 50 bar, for a period between 30 minutes and 20 hours. 14.Process according to claim 1, wherein the surface modification iscarried out in the presence of small amounts of a silylating agent. 15.Process according to claim 1, wherein the surface modification iscarried out in the presence of a catalyst.
 16. Process according toclaim 1, wherein prior to Step f), the surface-modified gel is subjectedto a solvent exchange.
 17. Process according to claim 1, wherein the gelis dried in Step f) under subcritical conditions.
 18. Process accordingto claim 1, wherein before the gel preparation, the sol is treated withIR opacifying agents and/or fibers.